The article entitled "Pr.sup.3+ doped fluoride fiber amplifier operating at 1.31 .mu.m" by Y. Ohishi, T. Kanamori, T. Kitagawa, S. Takahashi, E. Snitzer, and G. H. Sigel, published in Proceedings OFC' 91, paper PD2, San Diego 1991, discloses an optical amplifier using a fluoride glass optical fiber doped with praseodymium. The confinement of optical power in such a monomode fiber makes it possible to obtain a population inversion that gives rise to stimulated emission in the 1.26 .mu.m to 1.34 .mu.m spectrum range.
If the term "net gain" is used for the ratio expressed in dB of the signal at the outlet from the amplifying medium divided by the signal at the inlet thereto, then the net gain of the above-mentioned fiber amplifier is 5.1 dB at 1.31 .mu.m for a pumping power of 180 mW at 1.017 .mu.m.
However, it appears that presently-known fluoride fibers possess relatively poor mechanical properties. Firstly they are small in section having a radius of about 60 .mu.m, and secondly they are very sensitive to pollution from the outside environment, which makes them even more fragile.
An object of the present invention is to provide an optical amplifier in the 1.26 .mu.m to 1.34 .mu.m spectrum range having mechanical properties that are better than those of presently known doped optical fiber amplifiers.